This invention relates to liquid metal ion sources, and, more particularly, to alloys used as liquid metal ion sources for high vapor pressure metalloids.
Liquid metal ion sources provide high current density beams of metallic ions from a source having a small virtual source size. Such high brightness and small source size are required when the ion beam is to be focused with a high resolution of, for example, less than one micron spot size and utilized in applications such as fabrication of semiconductor microcircuits by ion implantation. The high current density and small virtual source size are achieved by emitting the ions from a substrate having a sharp point, such as the point of a needle. In one such approach, a needle is covered with a layer of liquid ion source metal, and a cusp in the liquid metal at the point of the needle is created by application of an electrostatic field. This tiny cusp then becomes the emitting source for the ions. As the ions are emitted from the source, more liquid metal flows from a reservoir down the needle to the cusp to replenish that evaporated.
In this type of ion source, a species to be implanted typically resides in a liquid alloy while in the reservoir and on the needle. This alloy must be heated to at least its melting point and remain in the molten state for long periods of time, typically at least 12 to 24 hours. When an alloy is held molten for this long period of time, species which have high vapor pressures can be lost from the alloy in significant amounts. This not only raises the pressure in the region of the needle to unacceptably high levels (ion focusing systems require pressure less than approximately 10.sup.-5 Torr) but also substantially impacts the alloy composition over time. This change in the composition of the ion source alloy over time can be highly significant in the fabrication of semiconductor microcircuits, due to the resultant change in composition and current density of species as it is implanted into the semiconductor. Additionally, the long period of contact between the molten alloy and the emission elements of the liquid metal ion source, including the reservoir and the needle substrate, can cause corrosion and failure of these elements.
The most straightforward approach to providing an evaporation source for a species is to provide it in its elemental, unalloyed form. However, many important and desirable dopant ion metalloids and metals for implantation of active areas of silicon microcircuits, such as arsenic, antimony, and phosphorus, have relatively high vapor pressures at their melting points. The melting points are also rather high, so that corrosion of the evaporation elements is possible when the liquid and evaporation element are in contact for long periods of time.
One alternative approach is to form an alloy of the desired evaporation species with other metal or metalloid constituents chosen to chemically bond with the desired evaporation metal or metalloid in the liquid state. This bonding tends to reduce the vapor pressure of the desired evaporation species by retaining it in the alloy. However, even with this technique for artifically suppressing the vapor pressure of the volatile constituents, the melting points of such alloys may still be sufficiently high to damage the ion source evaporation elements by corrosion.
Consequently, there exists a continuing need for better source alloys for use in liquid metal ion sources. Such alloys desirably would have low melting points, low vapor pressures of the volatile constituents when the alloy is molten, and would not corrode the components of the liquid metal ion source during long periods of operation. It would be further desirable if such source alloys could be used, in conjunction with a mass separator, as the source for different desirable dopant species. The present invention fulfills this need, and further provides related advantages.